1. Field of the Invention
The present invention relates to a paper post processing apparatus that performs sorting, stapling (binding), or the like with respect to sheets of paper that have undergone an image forming process.
2. Description of Related Art
There are occasions where one wants to perform binding processing (hereinafter referred to as stapling) or hole-forming processing (hereinafter referred to as punching) with respect to a relatively large number of sheets of paper onto which images have been transferred by an image forming apparatus such as a copier or a printer. On such occasions, it is convenient to use a paper post processing apparatus, that is, a so-called finisher that automatically performs predetermined post processing such as stapling or punching.
FIG. 10 is a vertical sectional view showing an example of the structure of a conventional paper post processing apparatus. In FIG. 10, the paper post processing apparatus 1 is detachably attached to the paper ejection side of an image forming apparatus (unillustrated) such as a copier, and is provided with a processing tray 2 that accommodates a stack of a plurality of sheets of paper P. When stapling is performed with respect to sheets of paper ejected one after another out of the image forming apparatus, the stack of sheets of paper P is temporarily stored in the processing tray 2. On the upper right side of the processing tray 2, there is provided a pair of carrying-in rollers 3, so that paper ejected out of the image forming apparatus is transported into the paper post processing apparatus 1 via the carrying-in rollers 3, and is then transported onto the processing tray 2 through a carrying-in entrance 4.
Also, the processing tray 2 is provided with a stopper 9 that is movable up and down along the processing tray 2. The stopper 9 is, when sheets of paper are carried in, placed in a stand-by state at the bottom end of the processing tray 2 so that one transfer-direction edge of the stack of sheets of paper P that have been carried in one after another onto the processing tray 2 via the carrying-in rollers 3 is supported by the stopper 9. In the vicinity of the stand-by position of the stopper 9, a stapling unit 5 is provided. The stapling unit 5 performs stapling with respect to a stack of sheets of paper P that have been aligned at one transfer-direction edge by the stopper 9. The stack of sheets of paper P that has undergone stapling is transported by the stopper 9 along the processing tray 2 to the upper portion of the paper post processing apparatus 1 to be ejected via stack ejection rollers 6 onto an ejection tray 7.
With regard to the paper post processing apparatuses structured as described above, various types have been proposed and commercialized that are capable of stapling a stack of sheets of paper P at two or more positions by moving the stapling unit 5 in directions substantially perpendicular to the transport direction of the stack of sheets of paper P (that is, in FIG. 10, in directions perpendicular to the plane of the figure). For example, JP-A-H10-338412 discloses a sheet processing apparatus that is provided with: a stapling unit arranged inside the main body of the apparatus; a stapler tray onto which sheets of paper with respect to which stapling is to be performed are temporarily carried to be stacked one on top of another; and a plurality of stopper members that determine where to stop the sheets of paper. Here, each stopper member is permitted to move independently between a stop position where it remains engaged with sheets of paper stacked on the tray and a retracted position where it remains away from the sheets of paper.
In JP-A-H9-142724, a paper post processing apparatus is disclosed that is provided with two stoppers for aligning an edge of sheets of paper, one disposed in a front side of and the other in a rear side of the bottom of a stapler tray, each stopper having a cutout window through which part of the surface of sheets of paper at which they are to be stapled is exposed, and furthermore, the cutout window of the stopper disposed at the rear part of the stapler tray is so shaped that the stopper does not interfere with a stapler when diagonal stapling is performed.
In JP-A-H9-86778, a stapling unit is disclosed that is provided with an angle changing mechanism that changes the angle at which diagonal stapling is performed by rotating a stapler around a rotation axis located in the vicinity of the stapling portion of the stapler and forward/backward moving means that moves the body of the stapler forward/backward between a stapling position in which it staples sheets of paper and a retracted position into which the stapler is retracted away from sheets of paper. Here, the angle and the position at which diagonal stapling is performed with respect to sheets of paper are freely adjustable, with as small a part as possible of the mechanism protruding out of the sheet corner.
However, in any of the apparatuses disclosed in JP-A-H10-338412, JP-A-H9-142724, and JP-A-H9-86778, since the stapler body and the stoppers are built as separate units, it is difficult, when the stapler or the stoppers is moved, to maintain an accurate positional relationship among them, and this tends to cause variation in the distance of a staple position from the edge of a paper stack, which is inconvenient. Also, drive means for moving the stoppers needs to be prepared separately, and this makes the apparatuses not only complexed in structure but also disadvantageous in cost.